In recent years, electronic devices and electric devices have been miniaturized, and accordingly, as coils attached inside such devices, those using an enameled wire with a rectangular cross section (rectangular enameled wire) are coming into use mainly in place of those using a conventional enameled wire with a circular cross section (round enameled wire). An insulating varnish is applied onto a conductor with a rectangular cross section (rectangular conductor) and then baked to form an insulating film on the rectangular enameled wire. The use of the rectangular enameled wire can reduce a gap between the enameled wires when the enameled wire is wound around a coil (that is, a space factor of the enameled wire can be heightened), and enables miniaturization of the coil. There has recently been an increasing demand for a rectangular enameled wire using, as its conductor, aluminum that is lightweight in place of copper that is conventionally used in order to achieve a weight reduction of the coil.
Aluminum, however, is oxidized in the air in an instant and thus an oxide film is formed on a surface of the aluminum conductor. Further, since aluminum is lower in melting point than copper, temperature when the insulating varnish is baked is limited. For these reasons and so on, the enameled wire using the aluminum conductor has had problems of poorer adhesion between the conductor and the insulating film and deterioration of processing resistance, as compared with an enameled wire using a copper conductor. Recent attention has been drawn to a polyimide-based resin varnish used as an insulating film material due to lower adhesion and processing resistance, in view of its low dielectric constant.